1. Field of the Invention
The present invention relates to a method for producing a semiconductor device using a thin film of a crystalline semiconductor, and particularly, to a method for producing a thin film transistor.
2. Description of the Related Art
Recently, much attention is paid on transistors which utilize a thin film of a semiconductor formed on a glass or quartz substrate. Such thin film transistors (TFTs) are fabricated by forming a thin film semiconductor from several hundreds to several thousands of angstroms (xc3x85) in thickness on the surface of a glass substrate or a quartz substrate, and then forming a transistor (insulated gate field effect transistor) using the thin film semiconductor.
TFTs are used in an application field such as that of an active matrix type liquid crystal display device. An active matrix type liquid crystal display device has several hundred thousands of pixels arranged in a matrix, and TFTs are provided to each of the pixels as switching elements to realize a fine and a high speed display. Practically available TFTs designed for an active matrix type liquid crystal display device utilize thin films of amorphous silicon.
However, TFTs based on thin films of amorphous silicon are still inferior in performance. If a higher function is required as a liquid crystal display of an active matrix type, the characteristics of TFTs utilizing an amorphous silicon film are too low to satisfy the required level.
Furthermore, it is proposed to fabricate an integrated liquid crystal display system on a single substrate by using TFTs; i.e., by realizing not only the pixel switching, but also the peripheral driver circuit with TFTs. However, a TFT using an amorphous silicon thin film cannot constitute a peripheral driver circuit because of its low operation speed. In particular, a basic problem is that a CMOS circuit is unavailable from an amorphous silicon thin film due to the difficulty in implementing a practical P-channel type TFT by using amorphous silicon thin film (i.e., the TFT using amorphous silicon thin film is practically unfeasible due to its too low performance).
Another technology is proposed to integrate other integrated circuits and the like for processing or recording image data, etc., on a single substrate together with the pixel regions and the peripheral driver circuits. However, a TFT using a thin film of amorphous silicon is too inferior in characteristics to constitute an integrated circuit capable of processing image data.
On the other hand, there is a technology of fabricating a TFT using a crystalline silicon film which is far superior in characteristics as compared with the one using a thin film of amorphous silicon. The technology comprises forming a film of amorphous silicon and then modifying (transforming) the resulting film of amorphous silicon to a crystalline silicon film by subjecting the amorphous silicon film to thermal treatment or to laser irradiation. The crystalline silicon film thus obtained by crystallizing the amorphous silicon film generally yields a polycrystalline structure or a microcrystalline structure.
As compared with a TFT using an amorphous silicon film, a TFT having far superior characteristics can be implemented by using a film of crystalline silicon. Concerning mobility, which is one of the indices for evaluating TFTs, a TFT using amorphous silicon film has 1 to 2 cm2/Vs or lower (in an N-channel type), but a TFT using a crystalline silicon film enables a mobility of about 100 cm2/Vs or higher in an N-channel type, or about 50 cm2/Vs or higher in a P-channel type.
The crystalline silicon film obtained by crystallizing an amorphous silicon film has a polycrystalline structure, and hence various problems attributed to the grain boundaries arise. For instance, carriers which move through the grain boundaries greatly limit the withstand voltage of the TFT. The change or degradation in characteristics easily occurs in high speed operation. Further, the carriers which move through the grain boundaries increase the OFF current (leak current) when the TFT is turned off.
In fabricating a liquid crystal display device of an active matrix type in a higher integrated constitution, it is desired to form not only the pixel region but also the peripheral circuits on a single glass substrate. In such a case, it is required that the TFTs provided in the peripheral circuit operate a large current to drive several hundred thousands of pixel transistors arranged in the matrix.
A TFT of a structure having a wide channel width must be employed to operate a large current. However, even if the channel width should be extended, a TFT using a crystalline silicon film cannot be put into practice because of the problems of withstand voltage. The large fluctuation in threshold voltage is another hindrance in making the TFT practically feasible.
A TFT using a crystalline silicon film cannot be applied to an integrated circuit in processing image data because of problems concerning the fluctuation in threshold voltage and the change in characteristics with passage of time. Accordingly, a practically feasible integrated circuit based on the TFTs which can be used in the place of conventional ICs cannot be realized.
An object of the present invention is to provide a thin film transistor (TFT) free from the influence of grain boundaries.
Another object of the present invention is to provide a TFT having a high withstand voltage and which is capable of operating large current.
A still other object of the present invention is to provide a TFT free from degradation or fluctuation in characteristics.
A yet other object of the present invention is to provide a TFT having characteristics corresponding to those of a TFT using single crystal semiconductor.
The above objects can be accomplished by a method for producing a semiconductor device according to the present invention, comprising the steps of, forming an amorphous silicon film on a substrate having an insulating surface, holding a metal element which accelerates (promotes) the crystallization of silicon in contact with the amorphous silicon film, forming a layer containing the metal element on the surface of the amorphous silicon film by heat treatment, forming a layer as a crystal growth nucleus by patterning the layer containing the metal element, forming a region substantially free of grain boundaries in the amorphous silicon film by crystal growth from the layer as the crystal growth nucleus, and forming an active layer by using the crystal-grown region which is substantially free of grain boundaries.
In the above process, the substrates having an insulating surface include a glass substrate, a quartz substrate, a glass substrate with an insulating film formed thereon, a quartz substrate with an insulating film formed thereon, and a conductor substrate with an insulating film formed thereon. Also in a constitution of a three-dimensional integrated circuit, an insulating surface comprising an interlayer insulating film and the like can be used as a substrate.
In the above process, the xe2x80x9cstep of holding a metal element which accelerates the crystallization of silicon in contact with the amorphous silicon filmxe2x80x9d can be performed by a constitution of FIG. 1A. In FIG. 1A, a solution containing nickel (a solution of nickel acetate) 104 is added to the surface of an amorphous silicon film 103 as a solution containing a metal element which accelerates the crystallization of silicon.
The state of holding a metal element which accelerates the crystallization of silicon in contact with the amorphous silicon film is realized in this manner. In this case, a solution containing the metal element is used, however, other methods for holding a metal element into contact with the surface of an amorphous silicon film can be employed. Such methods include forming a layer of the metal element or a layer containing the metal element on the amorphous silicon film by CVD, sputtering, or evaporation.
In the above process, the xe2x80x9cstep of forming a layer containing the metal element on the surface of the amorphous silicon film by a heat treatmentxe2x80x9d can include a step of FIG. 1B. In this step, baking at about 400xc2x0 C. is effected to form a silicide layer 105 containing nickel and silicon.
The step in FIG. 1C can be mentioned as the xe2x80x9cstep of forming a layer as a crystal growth nucleus by patterning the layer containing the metal elementxe2x80x9d. The step comprises patterning the silicide layer 105 to form layers 106 and 107 as crystal growth nuclei in the later step.
The steps of FIGS. 1D and 1E show the xe2x80x9cstep of forming a region substantially free of grain boundaries in the amorphous silicon film by crystal growth from the layer as the crystal growth nucleusxe2x80x9d. In FIG. 1D, laser light is irradiated while heating at 450 to 600xc2x0 C. to allow a crystal growth 108 to occur from the selectively formed nickel silicide layers 106 and 107 on the amorphous silicon film, and thereby forming monodomain regions 109 and 111 which do not include internal grain boundaries.
In the invention disclosed in the specification, at least one selected from the group of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au, or a combination of two or more thereof, can be mentioned as the metal elements for crystallizing silicon.
The region obtained substantially free of grain boundaries as a result of crystal growth is denoted as a monodomain region.
A monodomain region is free of point defects or planar defects which become grain boundaries, but has point defects to be neutralized. Thus, it is important that the monodomain region contain hydrogen or a hydrogen halide at a concentration of 0.001 to 5% by atomic for the neutralization. That is, hydrogen or hydrogen halide must be present at a concentration of 1xc3x971017 cmxe2x88x923 to 5xc3x971019 cmxe2x88x923 in the monodomain region to neutralize the point defects.
It is also a key that the monodomain regions contain the metal element for accelerating the crystallization of silicon at a concentration of 1xc3x971014 to 1xc3x971019 atomsxc2x7cmxe2x88x923. If the metal element should be present at a concentration not higher than the lower limit of the range defined above, the function of accelerating the crystallization would be achieved only insufficiently, and the required monodomain regions would be hardly obtained. If the metal element should be present at a concentration not lower than the upper limit of the range above, the semiconductive characteristics of the monodomain region obtained as a result would be greatly impaired.
The concentration referred above is defined as a minimum value based on the data obtained by SIMS (secondary ion mass spectroscopy). It should be noted, however, that the detection limit of SIMS at present for the metal element is 1xc3x971016 atomsxc2x7cmxe2x88x923. However, the concentration of the metal element can be approximated from the concentration of the metal element in the solution used for introducing the metal element. That is, the concentration beyond the limit of observed value by SIMS can be approximately calculated from the relation between the concentration of the metal element in the solution and the final concentration observed by SIMS for the metal element remaining in silicon film.
Two methods can be mentioned for the introduction of the metal element in effecting solid phase crystallization using the metal element for accelerating the crystallization.
One of the methods comprises forming an extremely thin film of the metal on the surface of the amorphous silicon film (or on the surface of the film provided under the amorphous silicon film) by a xe2x80x9cphysical methodxe2x80x9d such as sputtering or electron beam vapor deposition. In the methods above, the metal element is introduced into the amorphous silicon film by forming a film of the metal element in contact with the amorphous silicon film. In case this method is used, it is difficult to precisely control the concentration of the metal element to be introduced into the amorphous silicon film. Moreover, on an attempt to precisely control the quantity of the metal element to be introduced into the film by providing an extremely thin film about several tens of angstroms (xc3x85), it becomes difficult to form a film in a complete form.
More specifically, island-like film portions of metal element is formed on the surface of the deposition plane. That is, a discontinuous layer is formed. This problem can be overcome by, for example, molecular beam epitaxy (MBE) and the like. However, in practice, MBE is only applicable to a limited area.
In case crystallization is effected after forming the above discontinuous layer, each of the island-like regions function as a nucleus to proceed the crystallization.
By careful observation of the crystalline silicon film thus obtained by the crystallization from the island-like regions, amorphous components are found to remain in a great number. This can be observed by using an optical microscope or on an electron micrograph. Otherwise, this can be confirmed through the measurements using Raman spectroscopy. It is also confirmed that the metal components remain in aggregates. This is believed to occur because the metal components which function as the nuclei of crystallization remain as they are in the nuclei region.
The region in which the metal components partially remain as aggregates function as recombination centers for electrons and holes in the crystallized semiconductor regions. These recombination centers induce particularly undesirable characteristics such as an increase in leak current of the TFT.
Otherwise, a metal element capable of accelerating the crystallization of silicon can be introduced into an amorphous silicon film by utilizing a solution containing the metal element. This method comprises incorporating the metal element into the solution, and adding the resulting solution to the surface of the amorphous silicon film or to the surface of the base film on which the amorphous silicon film is formed by spin coating and the like.
Several types of solution can be used depending on the metal element to be introduced into the amorphous silicon film. Representatively, a metal compound available in the form of a solution can be used. Examples of the metal compounds usable in the solution method are enumerated below.
(1) In case of using nickel (Ni), the nickel compound is at least one selected from the group consisting on nickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickel chloride, nickel iodide, nickel nitrate, nickel sulfate, nickel oxide, nickel hydroxide, nickel acetyl acetonate, nickel 4-cyclohexylacetate, and nickel 2-ethylhexanate. Otherwise, nickel may be mixed with a non-polar solvent which is at least one selected from the group consisting of benzene, toluene, xylene, carbon tetrachloride, chloroform, ether, trichloroethylene, and Fleon.
(2) When iron (Fe) is selected as the catalytic element, an iron salt selected from compounds such as ferrous bromide (FeBr2.6H2O), ferric bromide (FeBr3.6H2O), ferric acetate (Fe(C2H3O2)3.xH2O), ferrous chloride (FeCl2.4H2O), ferric chloride (FeCl3.6H2O), ferric fluoride (FeF3.3H2O), ferric nitrate (Fe(NO3)3.9H2O), ferrous phosphate (Fe(PO4)2.8H2O), and ferric phosphate (FePO4.2H2O) can be used.
(3) In case cobalt (Co) is used as the catalytic element, useful compounds thereof include cobalt salts such as cobalt bromide (CoBr.6H2O), cobalt acetate (Co(C2H3O2)3.4H2O), cobalt chloride (CoCl2.6H2O), cobalt fluoride (CoF2.xH2O), and cobalt nitrate (Co(NO3)2.6H2O).
(4) A compound of ruthenium (Ru) can be used as a catalytic element in the form of a ruthenium salt, such as ruthenium chloride (RuCl3.H2O).
(5) A rhodium (Rh) compound is also usable as a catalytic element in the form of a rhodium salt, such as rhodium chloride (RhCl3.3H2O).
(6) A palladium (Pd) compound is also useful as a catalytic element in the form of a palladium salt, such as palladium chloride (PdCl2.2H2O).
(7) In case osmium (Os) is selected as the catalytic element, useful osmium compounds include osmium salts such as osmium chloride (OsCl3).
(8) In case iridium (Ir) is selected as the catalytic element, a compound selected from iridium salts such as iridium trichloride (IrCl3.3H2O) and iridium tetrachloride (IrCl4) can be used.
(9) In case platinum (Pt) is used as the catalytic element, a platinum salt such as platinic chloride (PtCl4.5H2O) can be used as the compound.
(10) In case copper (Cu) is used as the catalytic element, a compound selected from cupric acetate (Cu(CH3COO)2), cupric chloride (CuCl2.2H2O), and cupric nitrate (Cu(NO3)2.3H2O) can be used.
(11) In using gold (Au) as the catalytic element, it is incorporated in the form of a compound selected from auric trichloride (AuCl3.xH2O), auric hydrogenchloride (AuHCl4.4H2O), and sodium auric tetrachloride (AuNaCl4.2H2O).
Each of the compounds above can be sufficiently dispersed in the form of single molecules in a solution. The resulting solution is added dropwise to the surface on which the catalyst is to be added, and is subjected to spin-coating by rotating the surface at a rate in 50 to 500 revolutions per minute (RPM) to spread the solution over the entire surface. By previously forming a silicon oxide film at a thickness of 5 to 100 xc3x85 on the surface of the silicon semiconductor to enhance uniform wettability on the surface of the silicon semiconductor on which the film is formed, surface tension sufficiently prevents the solution from being scattered to form spots on the surface.
Further, the addition of an interface active agent into the solution realizes a uniformly wetted state on the surface of a silicon semiconductor having no silicon oxide film formed thereon.
In this method using a solution, a film of an organometallic compound containing a metal element is formed on the surface on which a film is to be formed.
The metal element which accelerates the crystallization of silicon is allowed to diffuse into the semiconductor in the form of atoms through the oxide film. In this manner, they can be diffused without positively forming (granular) crystal nucleus to uniformly crystallize silicon entirely. As a result, the metal element can be prevented from being partially concentrated or the amorphous component can be prevented from remaining in a large quantity.
The silicon semiconductor can be uniformly coated with an organometallic compound, and the resulting coating can be subjected to ozone treatment (i.e., treatment using ultraviolet radiation (UV) in oxygen). In such a case, a metal oxide film, and the crystallization proceeds from the resulting metal oxide film. Accordingly, the organic substance is oxidized and removed by volatilization in the form of gaseous carbon dioxide. Thus, a further uniform solid phase growth can be realized.
In case spin coating of the solution is effected by rotating at a low speed only, the metal component that is present in the solution on the surface tends to be supplied onto the semiconductor film at a quantity more than is necessary for the solid phase growth. Accordingly, after rotating at a low revolution rate, the spin coating is effected by rotating the substrate at 1,000 to 10,000 RPM, typically 2,000 to 5,000 RPM. The organometallic compound that is present in excess can be spun off by rotating the substrate at high rate, and the metal component can be supplied at an optimum quantity.
The quantity of the metal component to be introduced into the silicon semiconductor can be adjusted by controlling the concentration of the metal component in the solution. This method is particularly useful, because the concentration of the metal element to be finally introduced into the silicon film can be accurately controlled.
In the method of introducing the metal element using the solution, a continuous layer can be formed on the surface of the semiconductor (or on the surface of the undercoating thereof) without forming island-like regions of metal particles for the crystallization. Then, a uniform and dense crystal growth can be effected by a crystallization process by heat treatment or laser irradiation.
In the foregoing, an example of using a solution is described, but a similar effect as that obtained above can be obtained by forming the film by CVD using a gaseous metal compound, and particularly, a gaseous organometallic compound.
The method using a solution in forming a layer containing a metal element which accelerates the crystallization of amorphous silicon can be considered as a chemical method. The method for forming the layer by sputtering and the like as described above can be said as a physical method. The physical method can be considered as a non-uniform xe2x80x9canisotropic crystal growth methodxe2x80x9d using metal nucleus, whereas the chemical method can be considered as a method for uniform crystal growth, i.e., an xe2x80x9cisotropic crystal growthxe2x80x9d using a uniform metal catalyst.